1. Field of the Invention
The present invention relates to a sheet feeding apparatus, a sheet feeding method and an image formation system.
2. Description of Related Art
Conventionally, as a sheet feeding apparatus for supplying sheets to an image forming apparatus such as a copier and a printer, sheet feeding apparatuses of an air system have been known (see, for example, Japanese Patent Publication No. 4952524). As disclosed in Japanese Patent Publication No. 4952524, a sheet feeding apparatus of an air system (hereinafter referred to as “air sheet feeding apparatus”) includes a sheet storage section, a suction conveyance section, a floating air blowing section, a separation air blowing section, an outlet roller section and the like.
The suction conveyance section includes an endless conveyance belt having a large number of absorption ports, and an air absorption section installed inside the conveyance belt. The suction conveyance section is disposed above the sheet storage section. The suction conveyance section sucks, on the conveyance belt, sheets one by one from the sheet bundle loaded in the sheet storage section, and conveys the sheets. The floating air blowing section applies air to the sides of the sheet bundle from a direction orthogonal to the sheet conveyance direction (from the both ends of the sheet width direction) so as to float several upper sheets of the sheet bundle. The separation air blowing section applies air to a plurality of sheets sucked on the conveyance belt from the downstream side in the sheet conveyance direction and separates only the uppermost sheet. The outlet roller section is disposed on the downstream side of the suction conveyance section in the sheet conveyance direction, and configured to further output the sheet conveyed by the suction conveyance section toward the downstream side.
In the air sheet feeding apparatus, only the uppermost sheet which has been separated by the air of the separation air blowing section is sucked on the conveyance surface (suction surface) of the conveyance belt by the air absorption section. When the conveyance belt travels in this state, the uppermost first sheet is conveyed.
However, when the length of the sheet (the length in the sheet conveyance direction: sheet length) is longer than the length of the conveyance surface of the conveyance belt in the sheet conveyance direction, the sheet exceeding the conveyance surface of the conveyance belt hangs, and the hanging first sheet and the second sheet make contact with each other. As the sheet length increases, the range of the hanging part increases, and the contacting range with the second sheet increases. As a result, the frictional resistance between sheets increases, thus causing overlapped-sheets feeding in which the second sheet is sent together with the first sheet. In the case where air is applied to the second sheet to prevent the overlapped-sheets feeding, when the intensity of the air jetting is too high, the rear end of the first sheet which is not sucked on the conveyance surface bounces (sheet bounce), and deviates from the guide. Consequently, the sheet makes contact with the upper side structures and the like, and sliding resistance is generated.
In view of this, a technique has been provided in which a suction conveyance section is provided over the entire sheet surface, and the suction is performed in accordance with the sheet length so as to prevent the sheet bounce. For example, Japanese Patent Application Laid-Open No. 6-9083 discloses a technique in which an air absorption section installed inside a conveyance belt has a shutter for opening and closing an absorption port provided on the upstream side and downstream side in the sheet conveyance direction, and the absorption port is appropriately opened and closed with the shutter to selectively suck the sheet located at a position on the upstream side or downstream side in the sheet conveyance direction on the conveyance surface of the conveyance belt, and convey the sheet. In addition, Japanese Patent Application Laid-Open No. 9-216750 discloses a technique in which a slide valve is opened or closed in accordance with the length of the conveyed sheet, and an absorption force is generated only at a downward position of the sheet so that the sheet does not bounce.
However, in the techniques disclosed in Japanese Patent Application Laid-Open No. 6-9083 and Japanese Patent Application Laid-Open No. 9-216750 in which the entirety of the sheet surface is sucked in accordance with the sheet length to prevent sheet bounce, when sending of the uppermost first sheet with the conveyance belt is started, the first sheet is sequentially ejected out from the rear end (upstream) side of the conveyance surface of the conveyance belt in the sheet conveyance direction, and the second sheet is sucked with the absorption force on the conveyance belt at the conveyance surface from which the first sheet is ejected. Consequently, when the conveyance belt travels, overlapped feeding of the second sheet and the first sheet is caused. As such, it is necessary to partially stop the suction with a shutter and the like, or, stop the absorption fan itself for the purpose of preventing the suction of the second sheet.